1. Field of the Invention
The invention relates in general to a method of forming interconnections, and more particularly to a method of forming bonding pads.
2. Description of the Related Art
As the requirement on the complexity and precision of an integrated circuit design keeps increasing in order to reduce the feature size of a semiconductor device and increase the integration of an integrated circuit, a semiconductor device has to contain more than two metal layers to achieve the fabrication of high-density metal interconnects on a limited surface of a chip. As the design rules of a semiconductor device become finer and higher the requirements of lower resistivity and higher reliability increase, as well.
Since copper has better conductivity and reliability than aluminum, copper has become a new material used in the metal layers. During a conventional fabrication of interconnections, a copper pad is exposed in the air by a bonding pad window. The oxidation rate of copper is very high so that copper is easily oxidized to form copper oxide. That increases the resistance of the bonding pad and decreases semiconductor device reliability. Furthermore, the bonding pad adheres badly to a sweat joint with a welding line in the convention fabrication of interconnections. The yield of devices thus decreases from the bad adhesion.
FIGS. 1A to 1E are schematic, cross-sectional views showing a conventional process of forming a bonding pad. In FIG. 1A, a substrate 100 having a metal layer thereon is provided. An inter-metal dielectric layer 102 is formed on the substrate 100. A part of the inter-metal dielectric layer 102 is removed to form trenches 104, 105 and 106 therein.
In FIG. 1B, a conformal barrier layer 108 is formed on the inter-metal dielectric layer 102. A copper layer 110 is formed on the barrier layer 108 to fill the trenches 104, 105 and 106.
In FIG. 1C, a chemical-mechanical polishing process is performed to remove a part of the copper layer 110 until the inter-metal dielectric layer 102 is exposed. Copper lines 114, 115 and a copper pad 112 are formed within the trenches 104, 105 and 106.
In FIG. 1D, a cap layer 116 is formed on the inter-metal dielectric layer 102. A passivation layer 118 is formed on the cap layer 116. A defined photoresist layer 120 is formed on the passivation layer 118.
In FIG. 1E, a part of the passivation layer 118 and a part of the cap layer 116 are removed by the defined photoresist layer 120 to form a bonding pad window 122 and expose the copper pad 112. The photoresist layer 120 is removed.
According the process described above, since the bonding pad window 122 exposes the copper pad 112, the copper pad 112 is easily oxidized to form copper oxide. Resistance of the copper pad is thus increased to decrease device reliability. A welding line (not shown) is formed in the bonding pad window 122 to connect the copper pad 112. A preferred material of the welding line comprises gold. Since copper has bad adhesion with the welding line, a short occurs at the sweatjoint between the copper pad 112 and the welding line.